I don't think there is any easiest way to learn any new languages unless otherwise, one is ready to spend a good time and start as a newbie.  

Hi BP;

  The SnapDevelop IDE that now comes with the newer PB2019Rx and PB2021 installations is one of the best to use from the DataWindow perspective so that you can utilize some of your PB knowledge. Snap can build both .Net Framework and .Net Core entities as well and there are tutorials to follows for these.

  For C# in general though, I would recommend also installing the VS Express (free) IDE from Microsoft. There are "tons" of examples and tutorials for this IDE as well on the internet. Both Snap and VS can also build ready made C# solutions / skeletons which are great to study from the C# coding POV.

HTH ... Just my $0.02

Regards ... Chris

If you are looking to write a Web API, then I found the following Elevate 2021 video from Bruce Armstrong to be a game changer: https://www.appeon.com/developers/library/videos/developing-rest-api-snapdevelop.html .  

In terms of learning C# and .NET - Microsoft online documentation is very complete and well organized, but apart from the introductory tutorials (which are very good), the explanations are not necessarily the easiest to understand.  Lots of alternate explanations can be found by Googling, and they are often easier to understand.  

You have a daunting task ahead of you, but it is doable if you have a solid software development foundation.